1. Field of the invention
This invention relates to pipes that convey hydrocarbon fuel from deposits within the earth and more particularly to coatings for the interior surface of such pipes.
2. Description of Related Art
U.S. Patent Publication 2006/0017281 discloses the corrosive environment to which oil pipes are exposed, especially in the conveying of oil from underground deposits in the earth. Such oil well pipes also face the problem of plugging with asphaltenes, paraffin waxes and scale present in the oil. This Patent Publication solves these problems with a lining for the interior surface of the oil pipe, wherein the exposed surface of the lining comprises perfluoropolymer. Typically, the lining is composed of a primer layer adhered to the interior surface of the pipe and an overcoat adhered to the primer, perfluoropolymer preferably being in both layers, so that upon baking, the perfluoropolymer in the overcoat melt bonds to the perfluoropolymer in the primer layer [0010]. The primer layer also contains heat-resistant polymer binder which enables the primer layer to adhere to the oil pipe interior surface [0030]. The overcoat is essentially free of any ingredient other than the perfluoropolymer to provide the best non-stick surface [0045]. The lining in the form of coatings on steel substrates are tested for paraffin, asphaltene, and scale depositions, adhesion, and salt water permeation. The salt water permeation test is carried out in an autoclave containing stratified contact fluids, and is heated at 251 ° F. (122° C.) and maintained at 1026 psi (70.8 MPa) for 29 days. At the conclusion of the autoclave exposure, the autoclave is cooled to 104° F. (40° C.) over a period of several hours, and then the remaining autoclave pressure is released at a rate of about 100/psi/min, after which the impedance of the coating is tested. Reduction in impedance indicates permeability for the coating.
Environmental conditions to which the interior of pipes, i.e. downhole pipe that conveys hydrocarbon fuel from deposits in the earth vary geographically. The expression “pipe” includes tubes and tubing, these sometimes being an alternate expression for pipe. In this regard, downhole pipe may be called tubes, while pipe for surface conveying of hydrocarbon fuel, including undersea conveying, is usually called pipe. The term “pipe” also includes casing, that is a tubular structure that supports a hole and keeps the surrounding ground from collapsing. Hydrocarbon fuel includes liquid fuel, most notably oil, and liquid/gas fuel mixtures, most notably, unrefined natural gas, and mixtures thereof present in the same deposit. The environmental conditions at some drilling locations are much more severe than at other locations. These more severe conditions can be present both in the downhole pipe and pipe used to convey the fuel to storage or refining, i.e. fuel flow pipe.
To address these more severe conditions, more stringent tests have been proposed for coatings to be used on the interior surface of such pipe, to qualify the coating for testing in actual service. These tests are described under Test Methods herein as the Autoclave Test and the Acid Soak Test. The Acid Soak Test tests the impermeability of the coating to concentrated acid and requires the coating to be rather thick, i.e. at least about 50 μm in order to protect the substrate from corrosion. The Autoclave Test is similar to the autoclave test disclosed in U.S. Patent Publication 2006/0017281, with two major differences. First, the pressurization is much greater and second, a rapid rather than gradual decompression is used. The higher pressurization forces the test fluids present in the autoclave to penetrate into the coating, and the rapid decompression, results in the penetrated (absorbed) fluids (vapor) rapidly exiting the coating. A non-stick coating based on fluoropolymer as providing the non-stick character needed to avoid paraffin, asphaltene, and scale depositions inevitably requires the coating to be multilayer, at least one inner layer for providing adhesion to the substrate and at least one outer layer for providing the non-stick character to the coating. One or more of the test fluids penetrates all these layers during the Autoclave Test. The problem arises that the rapid exiting of the absorbed vapor during rapid decompression tends to cause blistering arising from the trapping of vapor within the coating, i.e. the absorbed vapor is unable to rapidly permeate out of the coating. Typically, the blistering occurs within an inner layer or at the interface between the inner layer and the outer layer. This blistering, depending on its severity, i.e. blister size, represents points of potential corrosive attack and coating failure in actual service using the coated pipe. The greater the coating thickness, especially with regard to the outer layer, the more difficult it is for the vapor to escape from the coating, leading to blistering from vapor entrapment. The Acid Soak Test also has the ability to cause blistering in the non-stick coating, and both Tests can attack the adhesion between layers and between the non-stick coating and the substrate.
The need to pass both the Autoclave Test and the Acid Soak Test presents the problem of conflicting requirements for fluoropolymer based non-stick coatings. The coating needs to be thick to stop corrosion in the Acid Soak Test, but needs to be thin in order to avoid blistering of the coating in the Autoclave Test. Reasonable adhesion of the coating to the interior surface of the pipe must also be maintained during both Tests.